The present invention relates to a method of producing a tin film on the inner surface of hollow component parts, such as tubes, tube joints or fittings, made of a copper alloy, such as red brass or yellow brass. The tin film serves to restrict the solubility of the copper in water.
Plumbing component parts of copper or copper alloys have proven successful for drinking water supply installations in homes. There is a limit for copper ion release of 2 mg/l to be maintained during twelve-hour stagnation of the water, thus the general object is to avoid direct contact between copper and water by using an inner coating. To ensure a suitable water quality for human consumption as per drinking water regulations, plumbing parts tinned on the inside are preferably installed.
A common method for internal tinning of copper tube is the chemical deposition of tin on the inner surface of the copper tube, as described, for instance, in U.S. Pat. No. 2,282,511. For this purpose, the inside of the copper tube is washed with a chemical tinning solution. The tin is then deposited through a simple chemical metal displacement (ion exchange). Copper ions dissolve from the base metal and an equivalent quantity of tin ions is simultaneously deposited from the tinning solution. This takes place under the influence of the potential existing between the tinning solution and the copper. An external voltage or an electric current is not required.
Chemical tinning is especially useful because of its simple method of operation at comparatively low cost for the equipment required. In addition, tin can be deposited on the inside of hollow component parts, which are generally quite difficult to access.
However, a disadvantage is the fact that the exchange of tin ions with copper ions takes place in dependence upon the available potential difference between the tinning solution and the copper component. Therefore, the rate of deposition decreases as a function of the thickness of the film already deposited, or the potential difference. This causes an irregular film formation when working with materials having non-homogeneous surfaces. The reaction stops as soon as the base metal is completely covered by the coating.
In this connection, it was discovered that, with lead-containing copper alloys, particularly with red or yellow brass, tinning success is hindered by the lead in the surface to be tinned. The lead which exists on the surface, and which, at up to 30%, is as a rule distinctly more concentrated than in the alloy as a whole, interferes with the ion exchange of tin and copper ions. This is detrimental to the adhesion of the tin film to the surface of the hollow component to be tinned. Also, it is usually only possible to achieve tin films having a thickness of less than 1 xcexcm. Furthermore, it was determined that migration of lead from the base material into the drinking water is essentially not decreased by the tin film.
In the case of plumbing components made of alloys of leaded copper for drinking water supply installations, the principle of decreasing the release of lead by treating the components with an aqueous reducing solution based on acid is generally known. Such proposals may be seen in European Patent Application No. 0 683 245 and in International Publication No. WO 97/06313.
The object of the present invention is to indicate a method for the internal tinning of hollow component parts made lead-containing copper alloys, which permits greater tin film thicknesses with improved adhesion.
According to the present invention, this object is achieved as follows. First, the lead content on the inner surface of the hollow component part is reduced by treatment with an acid-based aqueous reducing solution, such as, for example, a hydracid. Next, the hollow component parts are chemically tinned.